EP2514211B1 - Deflection system for a projection device, projection device for projecting an image and method for actuating a deflection system for a projection device - Google Patents

Description

The invention relates to a deflection device for a projection device, to a projection device for projecting an image onto an image field and to a method for controlling a deflection device.

In the prior art so-called Lissajous projectors are known in which mirrors are used which oscillate in two axes resonant or nearly resonant and thus sinusoidal. Due to the principle, these mirrors, also known as resonant scanners, are able to achieve much larger amplitudes than non-resonant scanners. larger Amplitudes are related to a scanning laser projection, equivalent to a higher optical resolution. For this reason, the Lissajous projection is of great interest for compact laser projectors, even if it has inhomogeneous illumination in both axes.

From the EP 1 419 411 B1 For example, a projection device is known in which the two scanning or driving frequencies of the two deflection or scanning axes differ by less than an order of magnitude, that is, f1 / f2 <10. The deflection or scanning system used for this purpose comprises means for fixing the scan frequencies in each axis to the frequencies f1 and f2, respectively. This ensures that there is a closed, stationary Lissajous figure, which repeats periodically. Moreover, for the Lissajous repetition frequency fr = f1 / n = f ₂ / m, where n and m are integers. This prior art proposes fixed frequencies for realizing the image projection. Thus, suitable means for frequency stabilization must be provided so that a fixed rational-frequency ratio f1: f2 = n: m persists.

Also the DE 10 2008 008 565 discloses a Lissajous projection method with two fixed frequencies whose largest common divisor forms the repetition rate of the Lissajous figure.

The patent US Pat. No. 7,580,007 B2 also proposes a Lissajous projection method with fixed frequencies f1 and f2, in which the Lissajous trajectory repeats after an integer number of horizontal oscillations.

WO 2006/063577 A1 describes another Lissajous projection method with a high-quality deflection unit and DE 10 2007 032801 A1 a projection method that adjusts the resonant frequency of the deflection unit by a second radiation source.

In the described Lissajous projection methods or projection apparatuses, which are based on the fact that the two scanning frequencies f1 and f2 of the resonance or resonance-operated axes of the deflection unit or the MEMS scanner are constant over time, the problem arises that the oscillation amplitude must be kept constant in time even if the resonant frequency of the resonant or the mechanical vibration systems, also called mechanical oscillators, by external influences, such as sudden temperature change, eg caused by changed laser power, changes. In order for the amplitude change associated with the change in the resonant frequency to remain low, a broad resonance curve, equivalent to a low quality factor, is required.

In Fig. 1 is an amplitude and a phase response over the frequency shown, with 1 is a damped resonance scanner or a low-attenuation attenuated unit with a quality factor of Q = 2,000 and with 2 the corresponding phase is shown. Here, the quality factor Q is defined as the ratio of resonance frequency to bandwidth, and the bandwidth is defined as the width of the resonance peak at the point where the attenuation reaches 3.01 dB. The projection devices described in the prior art are used only in conjunction with such strongly damped deflection units, in which the resonance curve is so flat that when changing the resonance frequency, for example due to temperature changes or the like, only a very small change in amplitude and phase change occurs, which may Increase the drive energy can be compensated quickly enough. This means that the known deflection units that have a low Quality factor, have good amplitude stability, good phase stability and also a wide frequency tunability. However, these known deflection units also have very crucial disadvantages. The high attenuation basically requires a much higher energy consumption, which is particularly disadvantageous for mobile use, such as in the mobile phone. Furthermore, the achievable amplitudes are limited with the usual forces available in microtechnology.

For the projection of very high-resolution images, such as in an HDTV resolution, not only extremely fast deflection units or resonators with deflection frequencies> 32 kHz are required, but also very large optical deflection angles. The term given is the so-called theta-D product, which is defined by the product of the diameter D of the deflecting element, e.g. a mirror plate, and one-sided mechanical scan or deflection amplitude theta or mechanical half-angle. For HDTV resolution, the required theta D product must be greater than 24 degrees * mm, e.g. at D = 1.5 mm and theta> 16 degrees, which corresponds to a total optical scan or deflection angle of> 64 degrees. These specifications can not be achieved with the strongly attenuated MEMS deflection units known in the prior art.

The invention has for its object to provide a deflection device which allows a high scan or deflection frequency and also provides a high theta-D product available.

This task is, but only partially (will be explained below), solved by a deflection, which has a high-quality deflection unit.

In the Fig. 1 is also the amplitude response of a little damped scanner or a little damped deflection and the associated phase response 4 is shown, it being recognized that the amplitude characteristic curve 3 has a strong resonance increase and the phase characteristic 4 has a large decrease, ie a large slope. A deflection unit, which is based on these curves 3 and 4, has a quality factor Q = 76,000. As can be seen from these curves, such a deflection unit at a fixed drive frequency or oscillation frequency even at low resonance frequency shifts quite significantly its amplitude, which can not be compensated with the available drive energy, ie that, for example, smallest temperature changes are sufficient to the To bring the deflection unit out of resonance. In this case, the drive signal with the fixed drive frequency would no longer accelerate, but already strongly decelerating effect. Therefore, the prior art Lissajous projection techniques with fixed frequencies can not be used in conjunction with very high quality resonators or baffles because they do not allow time stable image sizes or stable operation.

The following are some numerical examples for the curves according to Fig. 1 with the deflector with curves 3 and 4 being a wafer level vacuum-encapsulated biaxial gimbal-mounted micromirror scanner with mutually orthogonal axes having a figure of merit of Q = 76,000. The heavily dampened compared Low-level micromirror scanners with curves 1 and 2 and quality factor Q = 2,000 operate at atmospheric pressure, but have substantially the same dimensions as the high-grade mirror. The heavily attenuated scanner with Q = 2,000 changes its oscillation phase position by about 13 degrees with respect to the drive signal by an externally generated frequency shift of ± 0.5 Hz. A corresponding mirror provides a negative pressure operation and a quality factor of Q = 76,000 Phase shift of 154 degrees, when externally a frequency shift of ± 0.5 Hz is induced. With respect to the amplitude, such an externally induced frequency shift produces a reduction in amplitude of 14 dB in the slightly damped case, whereas in the damped case an amplitude change of only 0.06 dB occurs. Within the specified in the automotive sector temperature range from -40 ° C to + 85 ° C, for which the functionality of a device must be detected, depending on the mirror design and the resonant frequency very easily frequency shifts can be induced by a few hertz, which still would cause much larger phase and amplitude variations.

Thus, taking into account the above-mentioned embodiments, it is an additional object of the invention to provide a deflection apparatus for a projection apparatus for projecting Lissajous figures on an observation field which has a low attenuation and which can be used for a wide temperature range in resonance operation of the deflection unit , wherein the power consumption is lower than in the prior art deflection devices. Furthermore, the invention is based on the object, a method for driving a deflection device for a projection apparatus for projecting Lissajous figures, which allows operation of the deflector to resonate even if the resonant frequency of the deflector changes by external influence.

This object is achieved by a deflection device which has a quality factor of greater than 3,000 and whose drive device has a control circuit which is designed to regulate the first and / or the second drive frequency of the drive signals as a function of the measured phase position of the oscillations of the deflection unit. that the steep phase drop and / or the maximum amplitude of the oscillations of the deflection unit are kept in their resonance range, ie Phase and / or amplitude are kept substantially constant.

In a corresponding manner, in the method for controlling a deflection device, it is selected such that it has a quality factor of> 3,000 and that the phase position of the oscillations of the deflection unit is measured around at least one deflection axis and the first and / or second activation frequency depend on the measured phase position is controlled, that the phase position and the maximum amplitude of the vibrations of the deflection are kept in the resonance range.

By measuring the phase position and controlling the phase or the driving frequencies such that a constant amplitude in the resonance range is ensured, high quality scanners can be used, which have a large Gesamtablenkwinkel. The drive or control frequencies and thus at the same time the resonator oscillation frequencies provide the necessary Freedoms to keep the phases and the amplitudes preferably both deflection axes constant even under changing conditions of use or ambient conditions and thus to operate the deflection unit stable in resonance or near the resonance.

As a result, at the same time, the obtained Lissajous trajectory does not repeat periodically, i. a projection with non-stationary Lissajous trajectory is used in which the driving frequencies or resonant frequencies are not fixed and therefore do not have to be in an integer relationship to one another. The drive frequencies are variable according to the control device for phase and amplitude stabilization.

The features specified in the dependent claims advantageous refinements and improvements are possible.

The specifications for the permissible change range of the amplitude are determined by the properties of the deflection element and by the resolution of the observation field. For example, the range of change is specified as the reciprocal of the minimum resolution in an axis. In a definition using pixels, the amplitude should preferably change by less than a "pixel width". For example, in the case of a minimum resolution of 480 x 640 pixels, the amplitude of the deflector should change by less than 1/480 (0.00283) and 1/640 (0.00146). Preferably, the amplitude should change by less than 1%, more preferably by less than 0.5%, even better by less than 0.3%.

In a particularly preferred embodiment the phase-locked loop is followed by a line density control loop which influences the resonance frequency of one or both drive axes as a function of a current line density of the Lissajous trajectory given by the drive frequencies and possibly calculable such that the line density does not leave a predefinable tolerance range. For the determination of the instantaneous line density, the activation phase, ie the starting point or beginning of the oscillation, must also be taken into account. In this case, the usually electrostatically driven micromirror or the deflection unit can be selectively influenced with respect to the resonance frequency in different ways, namely by targeted temperature change, by targeted increase or decrease of the drive voltage of the deflection axes and / or by actively changing the spring stiffness. The temperature influence can be made inter alia by IR laser irradiation, as will be described below. An increase in the drive voltage of the deflection axes leads to a lowering of the resonance frequency, since the increased driving forces act as a softening of the spring suspension. The increase or decrease of the drive voltages can be made by superimposed on a DC voltage applied in addition to the drive potentials, wherein the strength of the resonance frequency shift can be influenced by the height of the amplitude of the DC voltage socket.

The targeted change in spring stiffness can be achieved with additional actuators attached directly to the spring, e.g. Fit the torsion spring and either compress or stretch it laterally.

Where appropriate with the line density control At the same time, the amplitude of the oscillation can also change, an amplitude control loop can be provided in addition to the phase-locked loop. In this case, the phase-locked loop has the highest priority and is permanently fast-reacting, while the line-density control loop and optionally the amplitude-locked loop are arranged downstream and react more slowly.

The line density control loop can perform its control using various information. For example, a list of discreetly stored unfavorable repetition rates, i. Drive frequency ratios (f1 / p, f2 / q) to be stored. However, such a list is not practicable in all cases, as it can be extensive if necessary and since an unfavorable tax claim would have to be allowed before it can be identified. Therefore, for example, it makes sense in another possibility to track the distance between the instantaneous repetition rate of the Lissajous figures and the next adjacent unfavorable repetition rate and to carry out a corresponding counter-regulation before the unfavorable case occurs. A PID controller can monitor the distance measure.

It is preferable that the deflection unit has a quality factor of> 20,000, preferably> 100,000. By using such high-quality resonator deflection units, the required large optical scan angles of greater than 60 degrees, for example with electrostatic deflection elements, can be achieved even at low drive voltages between 5 and 90 volts.

It is particularly advantageous that the deflection unit is vacuum-encapsulated, preferably at the wafer level. By this measure, i. operating at reduced ambient pressure (p <1 mbar) reduces the damping of the deflector vibrations. In this case, the quality factor can be selectively influenced by production technology by a targeted deterioration of the vacuum by means of a gas backfilling process. That is, first, minimal pressure is achieved by heating a getter (e.g., titanium) trapped in the mirror cavity and chemically bonding gas molecules. If the vacuum is too good (the pressure too low, or the quality factor undesirably high), then an inert gas filling can be carried out on the next wafer even before the final encapsulation and heating. These gas molecules can not be chemically bound by the getter and thus contribute further to the damping and thus to the quality factor reduction. A targeted vacuum improvement or increase in quality factor can only be achieved by using a getter with a higher getter capacity (better getter material and larger getter surface).

Preferably, the drive device has a measuring device for capacitive, optical, piezoresistive or piezoelectric measurement of the phase position. The phase position is determined from the amplitude information of the sinusoidal oscillations of the deflection, wherein preferably the zero crossing of the oscillation is used.

In a preferred embodiment, a radiation source for temperature stabilization is provided, wherein the drive device is designed, the radiation source for irradiating the deflection unit to turn on when the change of the first and / or second drive frequency occurring to control the phase position and the amplitude is greater than a predetermined value. This can be compensated for temperature changes.

It is advantageous that the line density control circuit comprises at least one radiation source directed onto the deflection unit, preferably an IR laser diode for influencing the temperature, and the control device or this control circuit is designed to control the power input produced by the radiation source, depending on the given value, the density of the Lissajous. Figures determining ratios of Ansteuerfrequenzen that are variable by the phase control to regulate. In this case, the ratios determining frequency shifts can be assigned predetermined and stored power entries, preferably in the form of a curve or a table or a programmed functional relationship or a mathematical function, in the simplest case, only the drive frequencies f1 and f2 but possibly also a number of parameters evaluates, eg f = fIR laser power (f1, f2, instantaneous projection laser power, instantaneous IR laser power, MEMS temperature, ...). The regulation can also be carried out by means of a PID controller. Without the option of trimming the frequency ratios, i. The temperature treatment would have a wealth of mirror chips are sorted out, because they have unfavorable resonance frequencies and Lissajous trajectories with consistently low line density.

The aim of the temperature treatment is to ensure that parallel to the amplitude stability, which by the phase control is ensured, at the same time the line density of the Lissajous trajectory based on the period of a field (typically 1/60 second) is optimal. In other words, the phase control keeps the mirror in resonance and stabilizes the amplitude. However, this regulation can lead to unfavorable line densities being set. Without the additional influence of the line density control loop, eg by the ThermoRegulation, there is no other way to change the line density.

The temperature influence on the deflection unit by irradiation of the deflection unit does not have to be permanently in use, but supplements the permanently existing phase regulation, ie the phase regulation must always be active, while the temperature influence need not always be active. The temperature influence could therefore have the character of a targeted disorder. But it can also be interpreted as a permanent scheme. That depends on the situation. Predictable fluctuations in the resonance frequency, which are caused, for example, by the changing image content, can be permanently compensated by infrared laser bombardment, for example by means of a look-up table. Even when writing or reading out the image memory, the energy input to be expected in the mirror can be calculated in advance and compensated in a corresponding manner by IR source. The counter tax information can be taken from a look-up table. Superimposed events, such as shock or vibration or externally induced unpredictable temperature changes must be compensated by additional thermoregulation depending on the elimination of the driving frequencies (and thus the line density) become. Thus, a disturbance must first occur so that it can be regulated by the control loop, unlike the predictable influence of the image content.

In a preferred embodiment, the deflection unit has at least one micromirror, wherein the total optical scanning angle indicative of the deflection of the mirror is> 30 degrees, preferably> 40 degrees and more preferably> 60 degrees. In this case, the micromirror may be a biaxial, gimbal-like micromirror suspended from torsion springs, but uniaxial mirrors arranged one behind the other may also be used. Usually, the mirror used in the deflection device will oscillate about two mutually orthogonal axes. However, angles deviating from 90 degrees between the scanning or deflection axes can also be included. Also, the inventive method or the deflection device according to the invention is not necessarily limited to two axes. Thus, a scanner with, for example, three or more torsional and / or spiral spring suspensions can also be used to realize a complex, densely packed Lissajous trajectory. The embodiments according to the invention are also not limited to a specific design of the MEMS scanner, for example, a cardan suspension of the mirror or a special type of drive, such as those with electrode combs. The prerequisite is always that the intended projection surface is scanned in sufficient speed and density by the projection beam. This could also be achieved by a MEMS actuator, which achieves the beam deflection not by reflection on a mirror, but by a refractive or diffractive element.

Preferably, the inventive deflection device is used for a projection device for projecting an image onto an image field, but the application bandwidth is not limited to such a projection device, but also includes scanning, sensory image detection tasks, such as in endoscopy or in scanning microscopy. In the projection device for projecting an image onto an image field, a modulation unit for modulating the intensity of the light beam depending on the image to be projected and the location of the light beam on the image field is additionally provided in addition to the deflection device. In a preferred embodiment, the radiation source for generating the light beam to be deflected comprises multi-color emitting laser diodes.

In the projection apparatus according to the invention, image information can in principle be transmitted at any time, ie pixels can be transmitted at all locations of the Lissajous trajectory. The images are thus preferably transmitted bidirectionally in each of the preferably two orthogonal scan axes. The modulation unit is preferably controlled by a control unit which reads the image data belonging to each mirror position, for example in the form of RGB pixel intensities in a fixed time clock, which is specified in a digital component, eg an FPGA or an ASIC, from an assigned image memory become. The modulation unit accordingly controls the readable intensity value for the modulatable RGB laser source. The projection device according to the invention is not limited to a fixed pixel projection rate, so it would also be possible to have a variable pixel projection rate which, for example, takes into account different scanning speeds and generates an equidistant pixel projection related to the scanning angle.

In the case of the projection device for temperature stabilization, one or more radiation sources are preferably associated with the deflection unit, which irradiates the deflection unit as a function of the intensity of the light beam determined by the image to be projected. In this way, the temperature fluctuations in the deflection unit caused by the intensities of the light beam can be compensated

Fig. 1

die Amplituden- und Phasengänge eines hochgedämpften Resonanz-Scanners niedriger Güte nach dem Stand der Technik und die Amplituden- und Phasengänge für einen bauähnlichen Scanner, sprich einer Ablenkeinheit auf MEMS-Basis hoher Güte,

Fig. 2

eine schematische Darstellung einer erfindungsgemäßen Ablenkeinrichtung mit einem Regelkreis,

Fig. 3

eine Aufsicht auf einen zweiachsigen, kardanisch aufgehängten Mikrospiegel mit Kammantrieben,

Fig. 4

einen Ausschnitt einer nicht geschlossenen Lissajous-Trajektorie, bei der das Verhältnis der Schwingungsfrequenzen in den zwei Achsen kleiner als die Auflösung des Pixelrasters ist,

Fig. 5

eine Darstellung entsprechend der Fig. 4, bei der das Verhältnis der Schwingungsfrequenzen näherungsweise identisch mit der Auflösung des Pixelrasters ist,

Fig. 6

einen schematischen Aufbau einer erfindungsgemäßen Projektionsvorrichtung mit erfindungsgemäßer Ablenkeinrichtung,

Fig. 7

ein Blockschaltbild eines Phasenregelkreises und

Fig. 8

ein detaillierteres Schaltbild der Ansteuervorrichtung für die Ablenkeinheit.

Embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. Show it:

Fig. 1

the amplitude and phase responses of a high-fidelity low-Q resonant scanner of the prior art and the amplitude and phase responses of a simplex scanner, that is, a high-quality MEMS-based deflector;

Fig. 2

a schematic representation of a deflection device according to the invention with a control loop,

Fig. 3

a view of a biaxial, gimbal-mounted micromirror with comb drives,

Fig. 4

a section of a non-closed Lissajous trajectory, in which the ratio of the vibration frequencies in the two Axes smaller than the resolution of the pixel grid,

Fig. 5

a representation according to the Fig. 4 in which the ratio of the oscillation frequencies is approximately identical to the resolution of the pixel raster,

Fig. 6

a schematic structure of a projection device according to the invention with inventive deflection device,

Fig. 7

a block diagram of a phase locked loop and

Fig. 8

a more detailed circuit diagram of the drive device for the deflection.

As already stated, are in the Fig. 1 Amplitude and phase responses of highly damped 1, 2 micromirrors and of high-quality 3, 4 micromirrors for resonant frequencies between 17.15 kHz and 17.17 kHz shown. In order to show the effects of external influences, for example temperature changes, a resonant shift is shown in dashed lines with the curve 3 'and the curve 4', ie the frequency has shifted by a slight value to lower frequencies. However, according to the prior art, the frequency of the drive signals remains at the previous value, ie the drive signals or the drive signals can act on the mirror in a decelerating manner, ie it no longer oscillates at the resonant frequency. Therefore, according to the invention, a phase control according to Fig. 2 proposed.

In Fig. 2 is schematic with a deflection unit illustrated, which comprises a biaxial, gimballed micromirror 31 as the deflection element 31. The deflection unit or the micromirror 31, whose drive is not shown in detail, is driven by a drive signal supplied by a drive device 32 for each axis with the frequencies f1 and f2 as drive frequencies. These drive frequencies f1, f2 should correspond to resonance frequencies of the mirror 31. So that the mirror 31 can be tracked even when its resonance frequency changes, the drive device 32 has a phase-locked loop 34 which adjusts the phase and thus the drive frequency of the drive signals so that the mirror operates essentially at resonance. For detecting the phase position, a measuring device 33 is provided which measures the sinusoidal deflection of the micromirror 31. In addition, a line density control loop is provided at 29, which corrects an unfavorable line density of the Lissajous trajectories which possibly arises as a result of the phase control and keeps within a predetermined tolerance range. This is done based on a deliberate detuning of either or both of the resonant frequencies.

Due to the high deflection or scanning speeds, the instantaneous mirror position can not always be measured discretely at any given time with sufficient precision and resolution. However, the phase position, in particular the zero crossing of the sinusoidal oscillations can be determined very precisely from continuously measured amplitude information. This can be detected by means of optical or capacitive, piezoresistive or piezoelectric sensors.

Optically, the position and phase angle of a resonant mirror can be detected in a time-resolved manner via a monitor laser beam and a position-sensitive 2D photodiode (PSD). Depending on the location of impact of the laser beam deflected at the mirror, different photocurrents are tapped off at the four deflection electrodes of the PSD and converted into a time-resolved XY position signal by current-voltage converter and subsequent difference formation, summation and finally quotient formation (difference divided by sum).

In the case of the piezoresistive measuring method, the mechanically induced stress in the torsion springs, which is generated during the torsional oscillation and which depends on the deflection angle, causes a change in the resistance of the sensor structures. This is usually evaluated by a Wheatstone bridge and provides an output signal proportional to the torsion angle.

In piezoelectric sensors based e.g. on aluminum nitride or lead zirconium titanate layers, the twist of the torsion spring produces a lattice change that causes a charge shift. The spatial charge change can be measured as a voltage proportional to the tilt angle.

In capacitive evaluation methods, one evaluates the time-varying capacitance between static and movable sensor electrode fingers which depends on the tilt angle. From the literature, a whole series of different evaluation methods is known. Frequently, so-called carrier frequency methods are used. For this purpose, a high-frequency modulated voltage is applied to the sensor comb structures. The movement of finger-shaped capacities generates a capacitive current whose waveform represents an amplitude modulation of the carrier signal. The amplitude modulation contains the information about the mirror movement and can be extracted by multiplication (mixing) and filtering.

In Fig. 7 a phase locked loop 34 is shown as it can be used in the drive device 32.

In general, all of the measurement methods described above are more or less subject to noise and interference, providing direct instantaneous utilization for projection location determination and pixel addressing corresponding to that for image projection (further discussed in connection with FIG Fig. 6 explains) required resolution impossible. Therefore, for example, a lock-in amplifier with a multiplier 35 and a low-pass filter 36 is used in the phase locked loop 34 in order to determine a precise phase difference signal. Furthermore, the control circuit 34 has a voltage-controlled (VCO) or numerically controlled oscillator (NCO) 37, which is connected downstream of the low-pass filter 36 and is connected to the multiplier 35. The multiplier 35 thus receives the signal from the measuring device 33, ie the feedback signal from the mirror 31 and the output signal from the oscillator 37.

In order not to control a phase difference zero, but to an arbitrarily adjustable value, a phase shifter between the output of the oscillator, which specifies the reference signal for the control loop, and the multiplier 35 must be provided, was omitted for the sake of clarity. The phase shifter becomes the reference oscillator signal (here the VCO signal before multiplication) so delayed that even a non-zero setpoint for the phase difference can be preset. In order to keep the mirror in resonance, this phase controller could be adjusted so that, taking into account all occurring delays in signal processing, a phase difference of, for example, 90 degrees between the drive signal and the mirror oscillation results.

The difference between mirror phase angle and phase angle of the drive signal can be determined with sufficient accuracy by means of the lock-in amplifier, by first multiplying the drive signal (reference oscillator) with the capacitive feedback signal, for example, and then low-pass filtering. At the output a DC signal is obtained according to trigonometric relationships, which has a phase difference (mirror phase to reference oscillator) proportional amplitude. If this output signal of the lock-in amplifier via an amplifier, not shown, (Gain) supplied to the input of the oscillator 37, a PLL control circuit (phase-locked loop) is obtained. With the help of this PLL, a constant phase difference between mirror and drive signal can be realized. In other words, the PLL can ensure that the mirror is always in resonance or defined near resonance, i. the drive frequencies f1, f2 coincide with the resonance frequencies of the mirror 31.

This means that the frequencies f1, f2 of the drive signals are not permanently constant, but may change in favor of the phase and amplitude stabilization. Furthermore, the resonance frequencies have of horizontal and vertical beam deflection no fixed integer ratio. As a result, the Lissajous trajectory is not stationary and there is generally no fixed repetition rate of the Lissajous trajectory. In the case of an image projection apparatus, this would mean that there is no fixed, predictable sequence of the image memory readout process since the scanning of the projection surface is not constant.

The temporal variation of the Lissajous trajectory results in the favorable circumstance that the unwanted speckle patterns that occur in laser projections are perceptibly reduced because the same but different scattering centers are not always hit on the projection surface. This has the consequence that the speckle patterns also vary and are averaged in the eye by overlaying and temporal integration. This is an advantage over all laser projection methods with fixed trajectories.

The in Fig. 2 shown phase-locked loop 34 is only schematically indicated for both axes, it can be provided for each oscillation axis, a phase-locked loop. Even if phase locked loops exist in each oscillation axis for stabilizing a phase difference between the excitation signal and the corresponding resonator oscillation, the oscillations of the two axes never form a stable phase relationship with each other because of the variable oscillation (oscillator) frequency.

In Fig. 3 is a biaxial, gimballed micromirror 5, as in Fig. 3 can be used. They are electrostatic away from the axis Comb drives 7 and near-axis comb drives 8 are shown, which can also be used as sensor electrodes. The mirror plate 5 is suspended via torsion springs 6 in a movable frame 9, which in turn is suspended by torsion springs 10 in a fixed chip frame 11. The frame 9 can be set in resonance by means of electrostatic comb drives 12, wherein the axis 9 has been omitted for reasons of driving or sensor purposes of the movable frame 9 for reasons of clarity.

In Fig. 6 an arrangement of a Lissajous laser projection device according to the invention is shown. This projection device has a deflection device according to Fig. 2 , where the vacuum encapsulated biaxial MEMS mirror scanner is designated by the reference numeral 22 here. As a radiation source, red, green and blue laser sources 18 are provided, whose light or radiation is parallelized by collimators 20 and formed by a beam combination system 21 to form a coaxial beam 15. This beam 15 is directed through the oblique glass cover 23 of the MEMS mirror scanner 22, onto the mirror. The deflected laser beam 16 biaxially illuminates a projection surface 24. Digital image data is supplied via an input 25 to a digital signal processing and control unit 13, in which control pulses corresponding to the image data are transferred to an analogue control unit 17 with the aid of which the emission of the laser sources 18 is controlled. 33, the measuring device 33 for measuring the deflection of the mirror of the deflection unit 23 is indicated, which is connected to the signal processing and control unit 13. The latter also drives an analog voltage amplifier 14. This forms part of the signal processing and control unit 13 according to the drive device Fig. 2 , wherein here only one Ansteuersignalleitung for both deflection axes with the frequencies f1, f2 is shown. The signal processing and control unit 13 in turn includes two control circuits 34 according to Fig. 7 ,

Furthermore, a fourth laser source 19 is provided, which is preferably a near-infrared laser source, e.g. a wavelength tuned to the absorption maximum of silicon laser diode, which can be used to keep the image data conditional fluctuations of the incident laser power on the mirror constant. In the illustrated arrangement, the laser source 19 radiates onto the non-mirrored rear side of the MEMS mirror scanner 22. In this way, the frequency of one of the two scanner axes or both axes can be influenced simultaneously with very low laser power.

The dependent on the mirror movement access to the image data in the memory of the signal processing and control unit 13 and the subsequent control of the laser sources 18 can proceed according to the following scheme. Using the optically, piezoresistively, piezoelectrically or capacitively obtained feedback signal, precise phase information can be obtained. As this feedback signal, as already related to Figure 7 described, generally have certain interference components and distortions, this feedback signal is further processed, preferably in the aforementioned lock-in amplifier, this further processing is included in the signal processing and control unit 13. At the output of the lock-in amplifier, a signal is thus obtained which is proportional to the phase difference between reference oscillator 37 (VCO see Fig. 7 ) and the feedback signal. Assuming that the mirror oscillates sinusoidally, the waveform of the mirror is synthetically reproduced in terms of frequency and phase. With regard to this mirror position signal, pixel coordinates for both axes can now be determined based on a fixed clock cycle (pixel clock), for example, and associated memory contents can be read out. All digital processes and components are part of the signal processing and control unit 13. After reading the memory contents (intensity values), the laser source drivers 17 can be correspondingly controlled on the basis of these contents and 18 pixels can be transmitted via the control of the laser sources.

The pixel projection does not provide a fixed, unchangeable fixed pixel raster, but a projection deviating from this raster, which should not mean that the image is distorted, but theoretically interpolated image pixels can also be set at the locations between two raster points , When a pixel is set is thus dependent on the pixel clock and not on the current location. The location determines only the pixel intensity to be transmitted from the image memory. Preferably, therefore, for projecting a pixel, not only an image memory cell, but all immediately adjacent pixels of the current location position is read out and interpolated in a focus-oriented manner. There is no time interval during which the pixel clock can not trigger image data projection.

In Fig. 4 and Fig. 5 are excerpts of an unclosed, ie non-stationary, Lissajous trajectory 27 shown. At those after Fig. 4 For example, the variable sampling frequencies or scan frequencies corresponding to the resonance frequencies of the mirror form a ratio that is smaller than the resolution of the pixel grid. Due to the non-integer ratio of these scan frequencies and because of the variable scan frequencies, the trajectory 28 no longer travels the same path as the trajectory 27 before. A stable image projection is possible even if slightly different (interlace) trajectories are produced during each scan become. It is important that the line density reached after 1/60 seconds sufficiently covers the image line resolution.

In Fig. 5 is a corresponding non-closed Lissajous trajectory 27, as in Fig. 4 shown where the ratio of the variable scanning frequencies is approximately identical to the resolution of a pixel grid 26. Again, the same trajectory is not covered in the second pass as previously in the trajectory 27th

While according to Fig. 4 a selection of the scan frequencies f1 and f2 takes place so that their quotient is less than the number of lines of the image to be projected and only a juxtaposition of several interlace images covers the entire image is in this representation after Fig. 5 the frequency ratio is adapted directly to the number of lines, so that already on the first pass each line contained in the picture is "scanned" or "read", "written" or "projected". On the second pass, each line is also read and projected again, but as illustrated the trajectory runs slightly offset, but not disturbing in the picture.

The phase control implies, as mentioned above, that the projection method is based on non-stationary Lissajous trajectories, wherein the drive or oscillation frequencies f1 and f2 of the two mirror axes are designed to be variable. As used herein, "variable" is understood to be variable within an interval determined by the possible shift of the resonant frequency of the deflection unit, e.g. due to temperature fluctuations, is given. The regulation can cause unfavorable line densities of the Lissajous figures, which are influenced by the current frequencies f1 and f2. So it is quite possible that an adjustment of one of the two frequencies at just 0.01Hz decides whether the projected image consists of 10 lines or a few hundred lines. Thus, favorable and unfavorable frequency intervals and frequency ratios can be defined. If the two frequencies f1 and f2 are close together, e.g. 24000Hz and 24057Hz, then there are relatively large favorable frequency intervals. However, if the frequencies f1 and f2 are far apart, about 500 Hz and 24000 Hz, then the intervals of favorable frequencies are narrower. In any case, such favorable and unfavorable ranges can be defined in advance depending on the desired resolution and derived therefrom limit values for the frequencies.

The Lissajous curve is known to be periodic if and only if f ₁ / f ₂ = p / q is a rational number, where p and q are integers and have no common divisor. The repetition frequency F of Lissajous curve is thus: $$\mathrm{F}={\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="imgf0001.png"\; state="\{\{state\}\}">f</figure-callout>}}_{\mathrm{1}}/\mathrm{p}={\mathrm{<figure-callout\; id="2"\; label="f"\; filenames="imgf0001.png"\; state="\{\{state\}\}">f</figure-callout>}}_{\mathrm{2}}/\mathrm{q}$$

In order to obtain a uniform illumination (high line density), the goal is to have F as small as possible. For a given f ₂ , this means that q should be as large an integer as possible. With regard to the choice of the frequency ratio f ₁ / f ₂ , one is therefore interested in a rational number p / q with the largest possible denominator. As small as possible F means that F should be smaller than the desired refresh rate f _B.

In the previous paragraph, stationary Lissajous trajectories were considered. Due to the phase control proposed in the invention, one does not have to deal with fixed frequencies f ₁ and f ₂ . Nevertheless, the preceding considerations are relevant for the design of the frequencies f ₁ and f _{2 of} the biaxial resonant scan mirror.

If f ₁ / f ₂ = p / q is rational, q gives (approximately) the number of nodes of the Lissajous figure in the x-direction or the y-direction. For q knots you need 2q lines that intersect. The number of pixels that can be resolved is thus q or 2q, depending on whether you count lines or nodes.

If the frequencies and thus their quotient fluctuate around 1% = 1/100, there is one worst-case rational number with a small denominator q, q is at best 100, usually much worse, typically between 10 and 20.

Therefore, it is part of this invention, the unfavorable Frequency relationships to avoid. The unfavorable frequency relationships can be avoided by the line density control circuit 29, in the further embodiment by thermoregulatory counter control or regulation, ie the deflection unit can be irradiated, for example, by the NIR laser source 19 so that the frequency f1 and / or f2 is changed. Of course, one or more further radiation sources can be provided.

In order to be able to carry out the thermal control or regulation in a targeted manner, it must be determined for each deflection unit after its production which ratios of the control frequencies lead to an unacceptable line density and must be avoided. For this purpose, these frequency ratios are stored in a memory of the drive device. In addition, the frequency deviations or shifts are stored in the form of curves or look-up tables as a function of the output power of the radiation source (s).

The control device or the line density control loop then monitors the frequency relationships or the instantaneous line density parallel to the image or Lissajous projection and, if unfavorable conditions occur due to the phase control, switches the radiation source on or off.

If, for example, the NIR laser source 19 or a corresponding one is selected, a certain amount of infrared laser power can be applied as base amount from the beginning, whereby the influence can be carried out in both directions, namely once by reducing the IR laser power and once by increasing the IR laser power.

The NIR laser source 19 can also be used to detect changes in the resonator frequencies, i. compensate the vibration frequencies of the MEMS mirror scanner 22 by, for example, changing the ambient temperatures. For this purpose, a predefined frequency range for the drive frequencies can be specified.

In Fig. 6 Of course, another location of the scanner 22 may also be selected to achieve a dominant effect on either the mirror resonant frequency or the resonant frequency of the outer moveable frame, or both. By applying two laser spots, once to the mirror, once to the outer movable frame, a separate simultaneous influencing of the resonance frequencies of both axes can be achieved based on two independently controllable laser sources.

In Fig. 8 is again more accurate after the deflection device Fig. 2 with the phase locked loop 34 of the Fig. 7 some of the new reference numerals are selected for clarity, although partially identical components are used as in these figures. 40 denotes the already described vacuum encapsulated 2D scanner or the deflection unit. For each deflection axis, a measurement signal 41, 42 is applied as a capacitive feedback to a current-voltage converter 43, 44 respectively, which are connected to A / D converters 45, 46, which convert the analog signals into digital feedback signals. The A / D converters 45, 46, the phase locked loops 47, 48 are connected downstream, each of the digital multipliers 49, 50, the digital low-pass filters 51, 52 and the oscillators (NCO) 53, 54 exist. Via D / A converters 55, 56 and analog voltage amplifiers 57, 58, the phase signals are supplied as drive signals to the drives of the deflection unit 40.

The respective output signals of the oscillators 53, 54, which correspond to the drive signals with the frequencies f1 and f2, are supplied to a unit 59 for the digital monitoring of the frequency ratio f1 / f2 and for the digital control of an IR laser source 60. In this unit 59, for example, the frequency ratios unfavorable to the density of the Lissajous lines are stored. If such an adverse frequency ratio would occur due to the phase control, the unit 59 drives the laser source 60 to irradiate the mirror 62 via an analog IR laser source driver 61. The necessary power input, e.g. is determined over the time of the driving of the laser source 60, depending on the frequency shift stored in the unit 59 or stored as a functional relationship. Thus, the unfavorable frequency ratio can be countered from the outset.

The described essential components are combined to form a control device 63 forming the control unit as FPGA or ASIC.

As already explained above, a particularly advantageous embodiment of the deflection device uses a two-axis MEMS mirror scanner whose resonant frequencies of the two orthogonal scan axes are above 16 kHz, differing only by less than 10% from one another by design execution. The advantages of such an arrangement are particularly suitable for the automotive sector, where it comes to realize a particularly shock and vibration-resistant projection system with very high resolution, such as for dashboard displays, dashboard displays or passenger entertainment. The shock resistance is achieved by the two very high and close to each other resonant frequencies. Unlike prior art Lissajous scanners with a very high frequency ratio of fast to slow axis, in the present invention there are typically no parasitic modes between the two payloads. Because of the relatively high two resonant frequencies, which differ only slightly, it is possible to simultaneously realize a wide frequency space by allowing the two scan frequencies to change without the possibility of low-density Lissajous trajectories.

Claims (19)

1. A deflection device for a projection apparatus for projecting Lissajous figures onto an observation field, which is configured to deflect a light beam about at least one first and one second deflection axis for generating Lissajous figures, comprising a deflection unit (30) for producing oscillations about the deflection axes and a control device (32) for producing control signals for the deflection unit (30), the control signals having a first and second control frequencies which substantially correspond to the resonant frequencies of the deflection unit, characterized in that
   the deflection unit (30) has a quality factor of > 3,000 and in that the control device (32) includes a first feedback loop (34) which is configured to regulate the first and/or second control frequencies in dependence on a measured phasing of the oscillations of the deflection unit so that the maximum amplitude of the oscillations remains in the resonant range of the deflection unit (30), whereby the control frequencies do not have a fixed integer number ratio, the control device (32) comprising a second feed back loop (29) which is configured to influence the resonance frequency of the first and/or second deflection axis dependent on a line density of the Lissajous figures given by the control frequencies in such a way that the line density is within a given tolerance range.
2. A deflection device in accordance with claim 1, characterized in that the first feedback loop is made to regulate the control frequencies so that the amplitude of the oscillations of the respective deflection axis changes by less than 1 divided by a minimal resolution of the observation field.
3. A deflection device in accordance with either of claims 1 or 2, characterized in that the second feedback loop (29) is configured to influence the resonance frequency of the first and/or the second deflection axis by varying of a given control voltage, by detuning the spring constant and/or by temperature influencing the deflection unit.
4. A deflection device in accordance with one of claims 1 to 3, characterized in that the first feedback loop (34) is made to regulate the control frequencies so that the amplitude of the oscillations of the respective deflection axis changes by less than 1%, preferably less than 0.5%, even more preferably less than 0.3%.
5. A deflection device in accordance with one of the claims 1 to 4, characterized in that the deflection unit (30) has a quality factor >20,000, preferably > 100,000.
6. A deflection device in accordance with one of the claims 1 to 5, characterized in that the deflection unit (30) is vacuum encapsulated, preferably on a wafer plane.
7. A deflection device in accordance with one of the claims 1 to 4, characterized in that the control device (32) has a measuring apparatus (33) for the capacitive, optical, piezoresistive or piezoelectric measurement of the phasing.
8. A deflection device in accordance with one of the claims 1 to 7, characterized in that at least one radiation source (19, 60) directed to the deflection unit (30) is provided for temperature compensation and the control device (32) is made to control the radiation source (19, 60) in an intensity modulated manner when the change of the first and/or second control frequency taking place for the regulation of the phasing and the amplitude is larger than a preset value.
9. A deflection device in accordance with one of the claims 1 to 8, characterized in that the second feed back loop (29) comprises at least one radiation source (19, 60) directed to the deflection unit (30) is provided for temperature influencing and the control device (32) is configured to control the power input caused by the radiation source (19, 60) in dependence on the present relationships of the control frequencies which determine the density of the Lissajous figures and which are changeably by the phase regulation.
10. A deflection device in accordance with claim 9, characterized in that the control device (32) or the second feedback loop (29) has a memory (59) in which an association table between the frequency shift and the temperature influencing is stored; or in that the control device (32) or the second feedback loop (29) is adapted in a technical programming manner so that the power input is controlled using a mathematical function.
11. A deflection device in accordance with one of the claims 1 to 10, characterized in that the deflection unit (30) has at least one micromirror (31), with the total optical deflection angle of the mirror being > 30 degrees, preferably > 40 degrees, even more preferably > 60 degrees.
12. A projection apparatus for projecting an image onto an image field having a deflection device in accordance with one of the claims 1 to 11 having projection radiation source (18) and a modulation unit (17) for modulating the intensity of the light beam of the projection radiation source (18) in dependence on the image to be projected and on the location of the light beam on the image field.
13. A projection apparatus in accordance with claim 12, characterized in that the projection radiation source for producing the light beam to be deflected includes at least one laser diode with multicolor emission, preferably a plurality of radiating laser diodes (18) with multicolor emission.
14. A projection apparatus in accordance with either of claims 12 or 13, characterized in that at least one radiation source (19) which can be intensity modulated for irradiating the deflection unit (30) is comprised and the control device (32) is made to control the radiation intensity of the radiation source (19) in dependence on the instantaneous radiation intensity of the projection radiation source (18).
15. A method for controlling a deflection unit for a projection apparatus for projecting Lissajous figures onto an observation field, wherein the deflection device having a deflection unit deflects a light beam about at least one first and one second deflection axis for producing the Lissajous figures and the deflection unit for producing oscillations about the deflection axes is controlled by a first and a second control frequency which are selected so that the deflection unit works in resonance, characterized in that the deflection unit of the deflection device is made so that it has a quality factor of > 3,000; and in that the phasing of the oscillations of the deflection unit is measured about at least one deflection axis and is regulated by changing the first and/or second control frequencies so that the amplitude of the oscillations is kept in the resonant range of the deflection unit, whereby the control frequencies for generating time varying Lissajoustrajectories do not have a substantially fixed integer number ratio and the resonance frequency of the first and/or second deflection axis is regulated dependent on a line density given by the control frequencies in such a way that the line density is within a given tolerance range.
16. A method in accordance with claim 15, characterized in that the control frequencies are regulated so that the amplitude of the oscillations of the respective deflection axis changes by less than the reciprocal value of a minimal resolution of the image of the Lissajous figures; and/or in that the amplitude of the oscillations of the respective deflection axis changes by less than 1%, preferably less than 0.5%, even more preferably by less than 0.3%.
17. A method in accordance with either of claims 15 or 16, characterized in that the deflection unit (30) is irradiated with electromagnetic radiation by a radiation source (19, 60), with the power input initiated by the radiation source being controlled in dependence on a frequency shift of at least one of the control frequencies occurring due to the phase regulation.
18. A method in accordance with claim 17, characterized in that the power input is controlled in dependence on preset relationships of the control frequencies determining the density of the Lissajous figures, with the frequency shifts determining the relationships being associated with preset power inputs, preferably in the form of a curve or of a table or mathematical.
19. A method in accordance with either of claims 17 or 18, characterized in that the power input is carried out when the change in the first and/or second control frequencies caused due to the phase regulation exceeds a preset value.

Images